The objectives are to discover the mechanism of acquired resistance to lice (in mice) and to distinguish that kind of resistance from native resistance and from the systemic kind of acquired resistance that occurs in tick infestation. Recent emphasis in our collaborative studies has been on histology and cytology at the several stages of reactivity to lice and to ticks and the effects of cortisone and of cyclophosphamide on cellular reaction. Differences in reactions between resistant and susceptible strains of mice are incorporated into the study program.